The turbine module of a gas turbine engine includes one or more arrays of blades, typically made of a nickel alloy, for extracting energy from a working medium fluid. Each blade comprises a platform with radially inner and outer surfaces, an airfoil that extends from the platform outer surface, and a root that extends from the platform inner surface. The root includes an attachment with "fir tree" teeth and may also include a damper pocket intermediate the platform and the fir tree attachment and defined in part by the platform inner surface. When installed in a turbine module, the fir tree teeth of each blade engage a corresponding fir tree slot in a rotatable turbine disk so that the platforms collectively define the radially inner boundary of an annular, working medium flowpath and the airfoil of each blade spans radially across the flowpath. Each damper pocket cooperates with the rim of the disk to confine a sheet metal vibration damper.
During engine operation, the airfoils and radially outer surfaces of the blade platforms are directly exposed to hot, working medium gases and are therefore susceptible to accelerated oxidation and corrosion. Accordingly, blade manufacturers apply a protective aluminide coating to both the airfoil and the radially outer surface of each platform. By contrast, the blade root and platform inner surface are normally left uncoated since they usually operate in an environment less conducive to accelerated oxidation and corrosion and since the presence of an aluminide coating could degrade the fatigue life of the attachment teeth and other highly stressed regions of the root.
The aluminide coating may be applied by conventional vapor deposition after the blade root and platform inner surface have been masked as described below. The masked blade is placed in a loosely covered coating vessel along with nuggets of an aluminum source material and a halide activator. The vessel and its contents are heated to an elevated temperature to vaporize the aluminum. Concurrently, an inert carrier gas (e.g. argon) is continuously pumped into the vessel to circulate the aluminum vapor and deposit the gaseous aluminum onto the unmasked blade surfaces where the deposited aluminum diffuses into the nickel alloy substrate.
Masking of the blade root and platform inner surface is accomplished with a coating box and a metallic masking powder. A typical coating box comprises a rectangular end plate with four walls extending perpendicularly therefrom to form a five-sided enclosure with an open end opposite the end plate. The end plate includes a window slightly smaller than the planform of the blade platform and at least partially bordered by ledges. The blade is mounted on the end plate so that the platform rests on the ledges and covers the opening and so that the blade root projects into the box interior. The masking powder is then introduced into the box through its open end and compressed around the blade root to completely envelop the root and shield the inner surface of the platform. The box and blade, with the blade root completely enveloped by masking powder and the airfoil projecting beyond the end plate, are placed in the coating vessel and the blade is coated as described above. During the coating cycle, the masking powder reacts with the coating vapor to prevent aluminum deposition on the masked surfaces.
The above described masking and coating procedures are effective when it is desired to protectively coat the flowpath exposed blade surfaces while precluding coating deposition on the root and platform inner surface. However, in some applications the damper pocket may operate in a temperature range conducive to hot corrosion (approximately 700.degree. C. to 900.degree. C.) and therefore may require a protective aluminide coating. Because the damper pocket is only moderately stressed, it is feasible to apply a protective aluminide coating to the damper pocket without incurring a detrimental reduction in fatigue life.
One way to selectively apply an aluminide coating to the damper pocket is to first mask and coat the blade as described above and to subsequently apply a coating precursor, in the form of a diffusible aluminide slurry, to the damper pocket surfaces. The blade is then once again heated to an elevated temperature to diffuse the aluminum content of the slurry into the substrate alloy. Although this method is effective, precisely applying the slurry to the selected surfaces is labor intensive and thus escalates the cost of coating the blade. Moreover, the added step of reheating the blade adds to the time required to complete the entire coating process.
Another way to achieve the desired selective coating application is to press an adherable aluminide coating tape into the damper pocket before mounting the blade in the coating box. The masking powder then masks the blade as before, except for those areas in contact with the tape. When the blade is heated inside the coating vessel to deposit the coating vapor on the airfoil and the platform outer surface, the aluminum content of the tape concurrently diffuses into the damper pocket. Although this method achieves the desired result, application of the tape is an exacting, time consuming process and the tape itself adds to the cost of coating the blade.
What is needed is a method and apparatus for selectively masking an article prior to depositing a coating on unmasked portions thereof, and particularly a convenient, cost effective and labor saving means for masking selected portions of a turbine blade root.